How to get UTC offsets from time zones of a particular region?

Question

I have a small list of time zones as shown below. It also has the IANA names for time zones.

How do I use javascript to get the UTC offset for these time zones for the current time? I can use libraries like dayjs also, but I prefer plain js. I could not find any answers for this. Here is the closest question I could find How to get UTC Offset from a timezone. But, it has no answers.

1 - Pacific Standard Time, America/Los_Angeles
    Expected utc offset = -420
2 - Eastern Standard Time, America/New_York
    Expected utc offset = -240

Answer 1

The Intl specification includes a timeZoneName option for the DateTimeFormat constructor that controls how the time zone name is formatted. One of the options is 'longOffset', which is suitable for this purpose. Also, a clean way to isolate that string is to use the formatToParts function.

function getCurrentOffset(timeZone) {

  // Create the formatter with the desired options.
  const format = new Intl.DateTimeFormat('en', {
    timeZone,
    timeZoneName: 'longOffset'});

  // Get the time zone name component, and slice off any leading abbreviation.
  const offsetString = format.formatToParts()
    .find(p => p.type === 'timeZoneName')
    .value
    .slice(3);

  // UTC and its equivalents must be handled directly, because Intl will return 'GMT'.
  if (offsetString === '') {
    return {offsetString: '+00:00', offsetMinutes: 0};
  }

  // Parse the hours and minutes from the result.
  const hours = offsetString.slice(0, 3);
  const minutes = offsetString.slice(4);
  const offsetMinutes = (-hours * 60) + ((hours < 0 ? 1 : -1) * minutes);

  // Return the ISO 8601 formatted offset string, as well as the total minutes offset.
  // Note, this returns minutes inverted, to match the behavior of the Date.getTimezoneOffset function.
  return {offsetString, offsetMinutes};
}

// some examples
console.log('America/Los_Angeles', getCurrentOffset('America/Los_Angeles'));
console.log('America/New_York', getCurrentOffset('America/New_York'));
console.log('Asia/Kolkata', getCurrentOffset('Asia/Kolkata'));
console.log('Pacific/Marquesas', getCurrentOffset('Pacific/Marquesas'));
console.log('Etc/UTC', getCurrentOffset('Etc/UTC'));